This disclosure relates to non-invasive determination of hemoglobin concentrations in blood, typically to non-invasive determination of total hemoglobin (THb) and hemoglobin fractions of a subject. This disclosure also relates to the apparatus, which is typically a pulse oximeter, and to a sensor and computer program product for the apparatus. Hemoglobin fractions here refer to the concentration percentages of different hemoglobin species.
Traditionally, hemoglobin measurements have been carried out based on in-vitro analysis of subject's blood. Measurement devices known as co-oximeters determine hemoglobin concentration from a blood sample by measuring spectral light transmission/absorption through a hemolysed blood sample at several wavelengths typically between 500 and 650 nm.
A major drawback related to co-oximeters is that the measurements are invasive, i.e. require a blood sample to be taken from the subject. Furthermore, the co-oximeters are rather expensive laboratory devices and require frequent service and maintenance.
One known technique for carrying out non-invasive in-vivo hemoglobin measurements is a so-called occlusion-release (OR) measuring technique, which is based on artificially induced changes in the blood flow of the patient. A typical OR based measurement device utilizes a ring-shaped cuff applied to the patient's finger. The device is further provided with a pressurizing arrangement to produce a state of temporary blood flow cessation in the finger by applying an over-systolic pressure and a state of transitional blood flow by releasing the over-systolic pressure. Measurement sessions are carried out during various states of blood flow and the blood absorption characteristics during the said states are analyzed to determine the concentration of a blood constituent, such as hemoglobin.
It is also known to combine the artificially induced changes in the blood flow with light transmission/absorption measurements at two or more wavelengths. These wavelengths typically include an isobestic wavelength (805 nm) and a wavelength at which water absorption is high (1310 nm or 1550 nm) to detect the concentrations of hemoglobin and water, respectively. It is also known to use the isobestic wavelength 805 urn and the water absorbing wavelength 1250 nm for measuring the total hemoglobin concentration using cardiac pulsating signals alone. Oxy-, deoxy-, carboxy- and methemoglobin fractions can be solved simultaneously with the total hemoglobin concentration using one set of equations and a total of six wavelengths.
Compared to invasive techniques, non-invasive optical hemoglobin or hematocrit measurements have clear advantages, which include the elimination of both painful blood sampling and the risk of infection. Furthermore, non-invasive measurements are simpler to carry out and require less training of the nursing staff.
However, there are also several drawbacks related to the above non-invasive techniques, as described below.
First, the devices that are based on stopping all or part of the blood flow are rather complicated since the optical measurement involves synchronized operation of the optical and pneumatic components of the measurement device.
Second, these measurements cannot be carried continuously, but a certain measurement period is required for each measurement. Typically, the measurement cycle is manually initiated, which makes the devices suitable for spot checks after the need for the hemoglobin measurement has been recognized based on symptoms of the subject/patient. Consequently, these non-invasive hemoglobin meters cannot be used for alarming of a sudden hemoglobin or blood loss.
Third, normal low-cost silicon detectors, which are used in standard pulse oximeters, can be used only in the visible and near infrared region, since their response ends at a wavelength of about 1100 nm. Therefore, more expensive detector technology, e.g, InGaAs detectors, must be used for enabling measurement of water absorption in the short-wavelength infrared region, such as at wavelengths around 1200-1300 nm. Coupled with the more expensive infrared emitters in this wavelength range, this need increases the cost of the sensor to many fold compared to silicon technology emitter and detector sensors.
Fourth, the measurements of total hemoglobin concentration and all hemoglobin fractions, including oxy-, deoxy-, carboxy- and methemoglobin percentages, require the use of two detectors as one detector cannot cover all wavelengths, from about 600 nm to about 1300 nm, required for the simultaneous measurement. Typically, a silicon detector is still needed for accurate carboxyhemoglobin reading, since carboxyhemoglobin absorbs light only below 700 nm, where a standard InGaAs detector cannot be used.
A hemoglobin measurement method is also known in which a theoretical relationship is formed, which is indicative of the effect of tissue on in-vivo measurement signals at the wavelengths of the apparatus. Hemoglobin concentration may be determined based on the theoretical relationship by requiring that the effect of the in-vivo tissue on the in-vivo signals is consistent for all wavelengths at which the in-vivo measurement is performed. This involves using a tissue model that includes hemoglobin concentration as one of the parameters, and adjusting the hemoglobin concentration in the model until consistency is reached. As the final result is to be searched for through iteration, this is actually a rather indirect measurement method and the result may still depend on other tissue parameters. Therefore, it would be desirable to obtain a more straightforward mechanism that eliminates the above drawbacks. It would also be desirable to be able to use the same wavelength set of the sensor for calculating both total hemoglobin and the hemoglobin fractions in blood, thereby to obtain a complete picture of the blood composition non-invasively and continuously with a compact and low-cost solution.